Numerous types of materials are often polished to impart various qualities, including, but not limited to, a desired amount of reflectivity and/or surface roughness. For example, in some cases it may be desirable to brighten a material to have a mirror-like reflectivity, such as for ornamental appearances. Further, it may be desirable to polish the material to have a smooth surface, such as for minimizing wear on a moving part.
Traditionally, materials have been polished using mechanical means. For example, steel articles requiring a high degree of luster and reflectivity are usually polished using mechanical means including, but not limited to, vibratory bowls, burnishing barrels, polishing lathes with belts, and tumbling media. However, such methods are not especially effective for parts having complicated shapes or which are otherwise fragile. Other methods are also known, including electrochemical methods for polishing. However, these methods also may not be suitable for complicated parts and otherwise require expensive equipment to achieve the desired results.
Further, many plating methods are known in the art, which can be used with or without polishing. Plating can be used to provide better corrosion and/or wear resistance. In this regard, the material is generally plated or coated with a nobler metal.
Conventional plating techniques include, for instance, galvanic plating and electroless plating. In galvanic plating, an externally applied electric current is used in conjunction with a solution that contains metal ions to thereby cause galvanic deposition of metal onto the surface of an object to be plated. Electroless plating, in contrast, connotes plating from a solution that contains metal or metal ions, without the necessity of external electric current application. Electroless nickel plating generally employs a plating solution that includes nickel ions (typically Ni2+ in a sulfate solution) and a reducing ion such as hypophosphite, and that typically further includes adjuncts such as complexing agents, buffering agents, and anti-pit agents. An object to be coated with nickel is introduced to the solution and the solution is brought to an elevated temperature, whereupon the reducing agent reduces the nickel ions to form nickel metal (Ni0). This causes the nickel metal to be plated onto the object.
A number of properties are desired of an electroless nickel plating solution. In many embodiments, an electroless nickel plating solution will impart a bright or semi-bright surface finish on the object to be plated. Generally, it is desirable that the electroless nickel plating solution be a low-pitting solution to mitigate against pits on the surface of the plated object. The solution should have good aging properties, by which is contemplated that, as the solution is consumed in use, the byproducts of the plating reaction, which include phosphates and other byproducts, do not cause degradation of performance, dulling of the finish, or the like.
Another desired property is shelf stability. In many embodiments, an electroless nickel plating solution is prepared in situ from two precursor solutions, one solution including nickels ions and the other including the reducing agent. In such embodiments, it is desirable that the two precursor solutions be themselves shelf-stable.
Many heretofore known electroless nickel plating solutions employ significant levels of lead or cadmium to assist in attaining one or more of the above-noted desirable properties. More recently, it has become desired to reduce or eliminate lead, cadmium, and similar heavy metals in light of toxicity and environmental concerns. Under the ROHS directive (with reference to directive 2002/95/EC of the European Parliament, Jan. 27, 2003), for example, lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers are limited or restricted in certain commercial applications. In light of this ROHS directive and similar standards that restrict the use of lead and cadmium, it is desirable to provide an electroless nickel plating solution that attains some or all of the above-noted advantages without necessitating the inclusion of lead or cadmium.